The present invention relates to an energy conserving limestone calcining system, especially as regards high calcium and/or dolomite limestone, and more particularly to a process and an apparatus for successive stage treatment of limestone to calcine the calcium carbonate, and/or magnesium carbonate, content substantially completely to calcium oxide, and/or magnesium oxide, and carbon dioxide with the use of comparatively less energy than otherwise, i.e., by incorporating certain thermodynamic principles of calcining limestone such that considerable energy is saved in the overall calcining process.
Lime (calcium oxide) manufacture involves thermally heating limestone (calcium carbonate) in a lime kiln so that carbon dioxide is liberated as a gas and solid particles of calcium oxide are left as the product, i.e. quicklime.
This can be illustrated by the following chemical equilibrium equation or reversible reaction: ##STR1## where, according to LeChatelier's principle, the degree of dissociation is dependent on the partial pressure of CO.sub.2.
Thus, if the partial pressure of CO.sub.2 on the right side of the equation is reduced, the chemical ratio of CaCO.sub.3 on the left side of the equation to CaO on the right side is also reduced and calcination occurs and proceeds to shift the reaction to the right side.
In a typical lime kiln, the reduction of the CO.sub.2 partial pressure is accomplished by supplying excess heat energy (.DELTA.) to the mass, to shift the reaction to the right side, therefore making the calcination of limestone an energy intensive process.
The heat energy source used may be a fossil fuel such as fuel oil, producer gas, natural gas, coal, coke, etc., but due to the comparatively high cost of any such fuel, the calcination of limestone is necessarily expensive to carry out.
Parenthetically, in this regard, quicklime prices in 1984 have been stated to be about $45 to $55 per ton (Chemical and Engineering News, Vol. 62, No. 31, July 30, 1984, page 16).
In practice, limestone is calcined in lime kilns that heat the limestone rock or stone lumps as they pass through the kiln. There are two primary types of kilns:
(1) upright cylinders or stationary vertical shaft kilns in which the stone is introduced at the top and allowed to pass slowly down through a heated zone and out the bottom; and
(2) rotary cylinders or horizontal rotary kilns in which the stone slowly passes through a rotating generally horizontal tubular apparatus;
in each case in countercurrent to hot heat imparting gases.
In both types of kilns, however, considerable excess heat is required to overcome the partial pressure of the attendant carbon dioxide and cause the calcination to go to completion.
This is due to the fact that limestone decomposes at comparatively moderate temperature according to the equilbrium equation I in what may be considered as a typical three component, two phase, and hence monovariant system, in which the dissociation pressure or vapor pressure of CO.sub.2 depends on the temperature.
In this regard, as is clear from the equilibrium equation I, calcium carbonate, as a first solid, decomposes or dissociates into calcium oxide, as a second solid, and carbon dioxide, as a gas. Since CaCO.sub.3 and CaO are both solids, their active masses or concentrations may be regarded as constant, such that the concentration of gaseous CO.sub.2 in equilibrium with solid CaO and CaCO.sub.3 must be constant at a given temperature. In turn, because the concentration of a gas is proportional to its pressure, it follows that at a given temperature the pressure of gaseous CO.sub.2 in contact with solid CaO and CaCO.sub.3 is also constant.
This temperature dependent constant pressure due to the gas formed, may be termed the dissociaton pressure or vapor pressure or partial pressure of the solid from which the gas is formed. The dissociation pressure of calcium carbonate in terms of carbon dioxide is well known as illustrated by Table 1, per the Handbook of Chemistry and Physics, 59th edition, 1978-1979, page F-90, as source, the list indicating the dissociation pressures and their related temperatures over a wide range, with the equivalent figures within parentheses having been calculated from the remaining corresponding values given in said source, in order to provide a convenient full comparison in both .degree.C. and .degree.F. temperatures, and in both mmHg and atmospheres absolute pressures:
TABLE 1 ______________________________________ Dissociation Pressure Of Calcium Carbonate (Vapor Pressure Or Partial Pressure of CO.sub.2 Over CaCO.sub.3) Temperature Pressure .degree.OC. .degree.F. mmHg atm. abs. ______________________________________ 550 (1022) 0.41 (0.0005394) 587 (1088.6) 1.0 (0.0013157) 605 (1121) 2.3 (0.0030263) 671 (1239.8) 13.5 (0.017763) 680 (1256) 15.8 (0.020789) 691 (1275.8) 19.0 (0.025) 701 (1293.8) 23.0 (0.030263) 703 (1297.4) 25.5 (0.033553) 711 (1311.8) 32.7 (0.043026) 727 (1340.6) 44. (0.057895) 736 (1356.8) 54. (0.071053) 743 (1369.4) 60. (0.078947) 748 (1378.4) 70. (0.092105) 749 (1380.2) 72. (0.094737) 777 (1430.6) 105. (0.13816) 786 (1446.8) 134. (0.17632) 795 (1463) 150. (0.19737) 800 (1472) 183. (0.24079) 819 (1506.2) 235. (0.30921) 830 (1526) 255. (0.33553) 840 (1544) 311. (0.40921) 852 (1565.6) 381. (0.50132) 857 (1574.6) 420. (0.55263) 871 (1599.8) 537. (0.70658) 881 (1617.8) 603. (0.79342) 891 (1635.8) 684. (0.9) 894 (1641.2) 716. (0.94211) 898 (1648.4) 760. atm. (1.0) 906.5 (1663.7) (874.76) 1.151 937 (1718.6) (1345.2) 1.170 1082.5 (1980.5) (6757.92) 8.892 1157.7 (2115.86) (14,202.12) 18.687 1226.3 (2239.34) (26,093.08) 34.333 1241 (2265.8) (29,771.44) 39.094 ______________________________________
Although it is seen from Table 1 that the carbon dioxide equilibrium pressure (100% CO.sub.2 atmosphere) reaches atmospheric pressure (i.e. 760 mmHg or 1 atmosphere absolute) at 898.degree. C. (1648.degree. F.), nevertheless as a practical matter, due to a tendency for the carbonate mass to superheat, it does not decompose rapidly at about 898.degree.-900.degree. C. (1648.degree.14 1652.degree. F.), but instead requires a temperature of at least about 910.degree. C. (1670.degree. F.) before reasonably rapid rate decomposition is reached.
Moreover, the decomposing mass of carbonate stone lumps must be retained at such temperature for a time sufficient to achieve substantial completion of the dissociation at atmospheric pressure, and of course, the liberated or generated carbon dioxide must be removed from the vicinity of the lumps to favor the shift to the right side per the equilbrium equation I.
Generally, the dissociation is accelerated at a temperature of at least about 1000.degree.-1100.degree. C. (1832.degree.-2012.degree. F.), and especially of at least about 1038.degree. C. (1900.degree. F.), or in more practical commercial scale terms is accelerated at a temperature of at least about 1093.degree.-1316.degree. C. (2000.degree.-2400.degree. F.), and especially of at least about 1204.degree. C. (2200.degree. F.). Indeed, the higher the temperature, the higher the rate of expulsion of the forming carbon dioxide gas from the stone lumps.
This is because the dissociation necessarily progresses from the outer zone of each lump toward its center in a more or less uniform circumferentially inward manner. Although the degree of natural porosity aids both the access of heat to the inner zones of the lump and the outflow of liberated carbon dioxide gas therefrom, higher temperatures than the theoretical are required in practice to enable the dissociation to penetrate such inner zones effectively.
In this regard, as the size of the lump increases, so also must its practical dissociation temperature and calcining residence time, in order for the dissociation to reach the center or core portion of the lump and for the temperature to cause the generating carbon dioxide to develop a sufficiently increased internal pressure in the interior of the lump for outward escape of the gas from the confines of such inner zones through the existing pores.
To the extent that water and organic impurities are present in the lump, they are volatilized and the organic impurities in turn are burned as the lump becomes heated, thereby preliminarily increasing the porosity of the lump for more effective dissociation of the carbonate within the crystal lattice of the lump once the carbonate dissociation is thereafter initiated.
The calcium oxide product is essentially infusible at the calcining temperatures used, and while it retains the general form of the original lump of starting calcium carbonate, it crumbles easily. As a result of the calcining, the crystalline mass has a tendancy to contract or shrink in volume, thereby narrowing its pores. This effect is more prounounced as the calcination temperature is increased.
In particular, if the carbonate starting material contains mineral impurities, especially silica, it should not be heated much above 1000.degree. C. (1832.degree. F.) because at this temperature, lime reacts with the silica and some of its other impurities, such as alumina and iron oxide, to form a fusible slag which seems to glaze over the oxide particles and adversely prevents subsequent reaction of the product with water, as in the forming of hydrated lime or calcium hydroxide.
Similar adverse effects occur from the absorption on the surface of the lumps of impurities, e.g. from ash and/or sulfur, in the fuel, i.e., in the case where the fuel or its combustion products come into direct contact with the lumps in the kiln.
The use of comparatively higher calcining temperatures, even in the case of relatively pure carbonate starting material, often adversely results in undue overburning of the surface layer of the lump, which leads to a reduction in the porosity, surface area and reactivity of the product.
Such undesired non-reactive forms are variously referred to as "hardburned" lime, "overburned" lime or "dead burned" lime, as the case may be, and these surface defective forms may contain various complex compounds on the surface of the lumps such as monocalcium and dicalcium silicates, calcium aluminates, calcium and dicalcium ferrite, calcium sulfate, and the like, which occlude the pores of the lumps and detract from the surface area and reactivity of the product, or may simply be overburned to the extent that the porosity and surface area of the lumps are so reduced that the product is sintered and essentially chemically inert.
Inasmuch as the burning or calcining of limestone is a simple thermal decomposition, in which it is generally necessary only to provide, as aforesaid, for a supply of heat and for the removal of the carbon dioxide which is formed, it would theoretically appear advntageous from the inherent temperature-vapor pressure relationship involved, simply to provide for sweeping away the carbon dioxide gas as rapidly as it is formed.
However, this advantage has not been realized in practice because in industrial scale operations, the lumps of limestone are comparatively appreciable in size, but more importantly because the crust or coating of calcium oxide as first formed on the outer zones of the lumps seems to act as a sponge to hold the vicinal carbon dioxide in contact with the reacting face of a given lump, by way of surface absorption thereat and/or superficial recarbonation, and thus to retard release of the carbon dioxide and full exchange via the pores with the inner zones of the lump for achieving more complete conversion of the latter to calcium oxide.
Consequently, the operation as a practical matter requires the full decomposition temperature demanded by the pressure equilibrium with the atmosphere to achieve substantially complete calcination as desired.
Where, on the other hand, the limestone is undesirably calcined insufficiently, e.g. at too low a temperature and/or for too short a residence time in the kiln, a comparatively large center or core of unconverted calcium carbonate is left in the lime lumps, and such core containing defective form is referred to as "underburned" lime.
Thus, other things being equal, limestone calcination is beset with the independent problems of minimizing adverse surface defects by the use of comparatively low calcining tempreatures, and at the same time of maximizing dissociation of the carbonate core by the use of comparatively high calcining temperatures, for a given residence time of the carbonate lumps in the kiln, so as to avoid the doubly undesirable result of a lump product having both an overburned surface and an underburned core.
As a general rule, higher burning temperatures and longer calcination times lead to a harder burned lime product of comparatively high shrinkage and density, and low porosity, surface area and chemical reactivity, whereas lower burning temperatures and/or shorter calcination times lead to a generally more desirable soft burned lime product of relatively low shrinkage and density, and high porosity, surface area and chemical reactivity, with a relatively small unreacted core and thus considered substantially completely calcined.
The degree of substantially complete dissociation of the calcium carbonate content to calcium oxide and carbon dioxide, and of any other carbonate also present, can be conventionally measured for example by the percentage by weight of loss on ignition (LOI), i.e. of CO.sub.2.
Inasmuch as lime is a poor heat conductor, and high local overheating should be avoided, a comparatively low temperature flame of large volume is usually employed for the calcining when conducted by direct heat exchange contact with the fuel and/or its combustion gases.
In the case of a vertical shaft kiln, the fineness of the limestone lumps used is limited by the necessity of keeping the comparatively static charge open enough by way of void spaces throughout its extent to induce a free passage of the flame and the products of combustion through the mass so as to avoid uneven burning or calcining of the stone. For this reason, small size stone lumps are normally avoided in a vertical shaft kiln since they do not provide large enough void spaces for adequate circulation of the flame and gases through the calcining mass.
On the other hand, since the charge is continuously tumbled on itself as it passes dynamically through the interior of a rotary kiln, smaller size limestone lumps are readily used therein, and the calcining residence time is therefore shorter than in the case of vertical shaft kilns.
For the shaft kiln calcination, the fuel consumption on an industrial sacle can, for instance, amount to about 4 to 6 million Btu/ton (1112 to 1668 kcal/kg) of limestone, e.g. in a modern central burner type vertical shaft kiln, and for rotary kiln calcination it can, for instance, amount to about 4.7 to slightly above 7 million Btu/ton (1307 to slightly above 1946 kcal/kg) of limestone, e.g. in a current type rotary kiln.
Larger size lump stone such as 4 to 8 inch lump stone is normally calcined in vertical shaft kilns, and smaller size lump stone such as that below about 4 inch lump stone, and particularly 0.25 to 2.5 inch lump stone or pebble lump stone, is normally calcined in horizontal rotary kilns.
Different size stones, of course, burn or are calcined at different rates since as earlier noted, the dissociation progresses from the outer to the inner zones of the limestone lumps. For uniformity of results, therefore, a uniform size or narrow graduation range of stone is best. While it would seem expedient to use the minimum possible size stone in a narrow graduation range to shorten the residence time in the kiln and conserve fuel energy and also obtain a uniform product, this must be balanced against the cost of crushing the raw stone from the quarry to achieve such narrow graduation range reduced size at acceptable levels of wastage in terms of fines, dust, etc.
It will be appreciated that the operation, including the calcining temperature and the kiln residence time selected, and in turn the quality and condition of the resultant oxide product, will necessarily depend not only on the content of impurities in the starting limestone and/or in any fuel or its combustion products coming into contact therewith, e.g. silica, alumina, iron oxide, etc., and on the lump size, but also on the extent to which associated magnesium carbonate (MgCO.sub.3) is present with the calcium carbonate, e.g. in dolomitic limestone or in magnesian limestone as compared to high calcium limestone. Magnesium carbonate, of course, decomposes at a lower temperature than calcium carbonate.
In this regard, as to commercially available forms of carbonate rock or stone of such types usable for lime production, in addition to an impurities content, mostly silica, of, for instance, about 1-3%, magnesian limestone or magnesite has a magnesium carbonate content (e.g. 5-20% MgCO.sub.3) generally intermediate that of high calcium limestone or calcite (e.g. of 97-99% CaCO.sub.3) and dolomitic limestone or dolomite (e.g. 40-43% MgCO.sub.3). Nevertheless, the magnesium carbonate compontent of dolomite seems to decompose at higher temperatures (e.g. 725.degree. C. or 1133.7.degree. F.; 760 mmHg) than in the case of natural magnesite.
Ordinarily, any carbonate rock having at least about 20% magnesium carbonate content is often roughly considered to be "dolomite" and that having at most about 2-5% magnesium carbonate is in turn roughly considered to be high calcium limestone. Because the proportion of magnesium carbonate varies among rock species, the dissociation temperature thereof correspondingly varies.
Consequently, upon calcination, due to the earlier dissociation of the MgCO.sub.3 content, the resulting MgO content is ordinarily hardburned to at least some extent prior to the ensuing dissociation of the CaCO.sub.3 content to CaO, inasmuch as the mass must be retained at a relatively high temperature throughout in order to achieve substantial completion of the CaCO.sub.3 dissociation for forming soft burned CaO.
This hard burned MgO content may be minimized to some extent by calcining the stone at minimum and constant temperature but for a longer residence time in the kiln.
As a rule, for less pure limestone (higher MgCO.sub.3 content), a calcining temperature of at least about 910.degree. C. (1670.degree. F.) and up to about 1000.degree. C. (1832.degree. F.) is typically used, and for limestone of higher calcium carbonate content or purity (lower MgCO.sub.3 content), a calcining temperature of between about 1000.degree.-1100.degree. C. (1832.degree.-2012.degree. F.) is typically used.
However, the operation may be carried out at still higher temperatures, e.g. up to about 1149.degree. C. (2100.degree. F.) or even up to about 1241.degree. C. (2266.degree. F.), as indicated in Table 1, or in more practical commercial scale terms up to about 1204.degree. C. (2200.degree. F.) or even up to 1316.degree. C. (2400.degree. F.), and in some cases up to 1482.degree. C. (2700.degree. F.), depending on the circumstances and the product results sought.
Aside from the foregoing, it will be further appreciated that as the calcining temperature increases, so does the tendency of the limestone impurities and those in any direct contact heating fuel used, such as ash constituents and sulfur in coal or coke, and sulfur in fuel oil, to react not only with the forming lime content itself in the lumps, but also with the refractory lining of the kiln. Where higher calcining temperatures are contemplated, e.g. above about 1100.degree. C. (2012.degree. F.) and especially above about 1149.degree. C. (2100.degree. F.), more expensive refractory linings are required for the kiln to minimize the effects of this tendency.
In fact, local overheating or hot spots not only lead to nonuniformity in the calcining product, but also often cause accretions and premature failure of the refractory lining, especially in rotary kilns using coal as fuel, requiring premature shut down for repairs. Due to the nature and thickness of such linings, start up and shut down of the kiln operation, which is normally a twenty four hour a day continuous operation, must be carried out by slow incremental heating and cooling, respectively, in order to minimize the adverse effects of thermal expansion and contraction in damaging the integrity of the refractory material itself, since any sudden change in temperature can cause the lining to disintegrate.
As is known, these refractory linings must be reasonably thick in order to provide structural protection of metal parts and at the same time minimize heat loss from the kiln. Their repair and replacement further burdens the cost of lime production, since apart from the cost of such repair and replacement the kiln must be kept out of service during the interim, and for each instance of down time the kiln must be slowly cooled and in turn slowly reheated, thereby consuming extra time and extra fuel without any benefit in terms of limestone calcining throughput.
The usual vertical shaft kiln generally consists of an upright steel cylinder for strength and prevention of gas leakage, which is lined with fire brick as refractory material. It may be fired for instance with producer gas, e.g. laterally or centrally supplied to a throat portion near the bottom of the shaft, such that a major portion of the air for combustion is upwardly supplied through the bottom. The relatively large lump limestone is continuously fed to the top of the shaft, and is preheated by the upwardly rising hot gases and then calcined at the combustion zone in the vicinity of the throat portion supplied with the producer gas and where the gas meets the incoming air, already itself preheated by the descending mass of hot lime which exits from the bottom of the shaft. The products of combustion and liberated carbon dioxide are discharged as spent gases for instance laterally at a point near the top of the shaft.
On the other hand, if coal is employed as the shaft kiln fuel, e.g. in a fire box of an indirect fired kiln, this same air supply principle may be used, such that secondary combustion may be induced in the body of the kiln by introducing some of the air through the bottom of the kiln. This prevents an excessive temperature in the region adjoining the fire box, and makes possible a kiln of much larger cross section than otherwise.
In particular, the use of producer gas, which is highly diluted with nitrogen and added steam, generally insures a voluminous flame which penetrates throughout the entire central body of the kiln. The gas flame may be made of even larger volume without dropping the temperature in the kiln, by the expedient of recycling to the intake of the producer for the producer gas some of the spent gases exiting from the top of the kiln since the carbon dioxide in this gas is partially reduced by the well known endothermic reaction to carbon monoxide in the hot body of the producer, and thus enriches the fuel gas thereafter fed to the kiln.
However, at the same time, this expedient undesirably not only consumes heat energy in the upstream producer to compensate for such endothermic reaction, even though it may not drop the temperature in the downstream kiln, but also and more importantly enriches the kiln with that part of the carbon dioxide in such recycled spent gases which is not so reduced to carbon monoxide, to the relative detriment of the carbon dioxide partial pressure and the desired shift to the right side per the equilibrium equation I.
Furthermore, aside from recycling CO.sub.2 containing exhaust gases directly or indirectly to the lime kiln, and aside from the relative cost of solid fuel such as coal and coke (e.g. of calorific value of about 10,000-16,000 Btu/lb.), and liquid fuel such as fuel oil (e.g. of calorific value of about 16,000-18,000 Btu/lb.), as compared to gaseous fuel such as natural gas (e.g. of calorific value of about 900-1200 Btu/ft.sup.3), producer gas (e.g. of calorific value of about 150-300 Btu/ft.sup.3 from coal), etc., it will be appreciated that, other things being equal, while the combustion conditions may be adjusted for substantially complete burning, on the one hand, of solid or liquid fuel theoretically to CO.sub.2 and H.sub.2 O in the combustion gases, nevertheless, on the other hand in the case of gaseous fuel, even where substantially completely burned as well, the combustion gases will inherently contain a higher CO.sub.2 content in all cases but natural gas, due to the significant presence of extraneous CO.sub.2 in the synthesized starting gas.
This extraneous source of CO.sub.2 will also independently adversely influence the CO.sub.2 partial pressure in the lime kiln and lead to relatively less efficient results in the same sense as occurs in recycling spent gases to the kiln.
Moreover, in the case of all such fuels, including those synthesized gaseous fuels containing extraneous CO.sub.2, although in the primary reaction the carbon content of the fuel is oxidized to CO.sub.2, and any hydrogen content of the fuel is oxidized to H.sub.2 O, an undesired secondary reaction often occurs as well, in which part of the CO.sub.2 present in the combustion gases reacts with additional carbon, e.g. where there is relatively an insufficiency of oxygen and/or an excess of hydrogen in the combustion system, and is thereby reduced to CO.
As earlier noted, the reduction of carbon dioxide to carbon monoxide involves an endothermic reaction, and this decreases the kiln temperature, directly or indirectly, as well as causes potentially usable heat values to be lost with the spent combustion gases exiting from the kiln. As a result, there is a corresponding reduction in the thermal efficiency of the calcining operation.
In contrast to the vertical shaft kiln which is normally used for burning or calcining relatively large lump stone, a horizontal rotary kiln is used normally for burning relatively small lump stone, and generally consists of a slightly inclined steel tube, which is also lined with refractory material. It may be fired for instance by gas as above discussed or by powdered coal or by atomized and vaporized fuel oil. Because the stone is of relatively smaller size, it is generally calcined in a much shorter time than in the case of the vertical shaft kiln, as earlier noted.
It will be appreciated that modern day rotary kilns can readily burn coal, gas or atomized and vaporized fuel oil with equally good calcining results, whereas most existing vertical shaft kilns are normally more restricted in terms of their design features to burning gas or fuel oil, few being able to accommodate solid fuel with any degree of efficiency except as producer gas source.
Theoretically, based on the equilibrium equation I, the complete conversion of calcium carbonate (mol. wt. 100) to calcium oxide (mol. wt. 56) and carbon dioxide (mol. wt. 44) causes a 44% reduction in weight of the starting limestone solids content (theoretically 44% LOI), and provides a corresponding 56% reduced weight quicklime product. Although the infusible quicklime product obtained generally retains the form of the original lumps of starting limestone, as earlier noted, its bulk is 44% lighter in weight and, other things being equal, is correspondingly more porous internally or less dense due to the loss of the carbon dioxide content from the crystal lattice of the starting solids material.
In the case of the proportionate content of magnesium carbonate (mol. wt. 84.3) also present in the starting carbonate rock or stone, theoretically its complete dissociation to magnesium oxide or magnesia, e.g. periclase (mol. wt. 40.3), and carbon dioxide (mol. wt. 44) causes a 52.2% reduction in its weight content (theoretically 52.2% LOI), and provides a proportionate 47.8% reduced weight final solids content. Hence, the greater the magnesium carbonate content in the starting carbonate rock or stone, the greater will be the weight loss after calcination.
However, in all cases, the calcining will only proceed to substantially complete decomposition of the limestone when the partial pressure of the produced carbon dioxide is maintained constantly below the equilibrium pressure at the corresponding temperature, and this usually requires a calcining temperature over 898.degree.-900.degree. C. (1648.degree.-1652.degree. F.) to drive the action forward, i.e. at normal atmospheric pressure, such as a temperature of from about 1000.degree.-1100.degree. C. (1832.degree.-2012.degree. F.), as aforesaid.
In actuality, because of the various practical factors involved, complete calcination of any commercially available carbonate rock will never provide a product which is 100% free of carbon dioxide, such being essential unattainable even under strict laboratory conditions.
Instead, some superficial recarbonation about the lump exterior will normally occur, due to the carbon dioxide rich atmosphere in the kiln traceable to the carbon dioxide liberated from the lump itself and that in any attendant combustion gases in direct contact with the lumps in the kiln, so that at least about 0.1-0.2% CO.sub.2 will always be present in the calcined lump. However, this recarbonated surface coating in fact limits further recarbonation since it occludes the surface pores of the lump sufficiently to inhibit recarbonation of attendant carbon dioxide gas to a further extent within the inner zones of the lump under the normal calcining temperatures involved.
Also, even in the case of soft burned lime, a small calcium carbonate core, e.g. about 0.25-2%, normally remains in the center of the lump product, as attempts to achieve dissociation completely of such core would otherwise cause the lime to be overburned or sintered to dead burned or nonreactive condition.
Hence, as contemplated herein, substantially complete calcining embraces the concept of carbonate rock dissociation to such an extent that the oxide particle product possesses at most about 0.1-0.2%, or less preferably at most up to about 2%, CO.sub.2 as particle or lump surface recarbonated coating, and at most about 0.25-2%, and less preferably at most up to about 5%, unconverted carbonate as core material.
Various specific proposals are known for vacuum calcining of carbonate rock and the like, which employ direct or indirect heat exchange systems using solid, liquid or gaseous fuels or electrical energy, generally in arrangements of the vertical shaft kiln type. However, these may be deemed obsolete in terms of present day energy costs since they inefficiently consume prohibitive quantities of thermal energy in attempting to achieve such vacuum calcining operation, as is clear from the following.
McTighe (U.S. Pat. No. 736,869, issued 1903), per a first embodiment, calcines a batch of limestone, under vacuum and by indirect heat exchange, in a closed vertical furnace to make carbon monoxide. The furnace contains a vertical central tube filled with coke or coal and flow connected at its top end with a series of surrounding like tubes filled with limestone, all the tubes being inserted in a bed of solid fuel disposed in the furnace. The furnace has a combustion products flue at its upper end above the level of the bed fuel and an ash pit at its lower end separated from the bed fuel by grates and presumably freely supplied with combustion air for upward flow through the grates. A vacuum pipe is connected to the bottom end of the central coke and coal tube for applying a vacuum to all of the tubes.
Upon burning the bed fuel in the McTighe first embodiment, the limestone and coke or coal in the respective tubes are heated by indirect heat exchange to incandescence at about half the temperature normally used, such that the entire limestone charge is calcined under vacuum and the liberated carbon dioxide is drawn from the limestone tubes through the central tube for endothermic reaction therein with the coke or coal to form carbon monoxide (which thereby consumes heat and produces a cooling effect), the carbon monoxide being recovered via the vacuum pipe as a valuable product, as opposed to carbon dioxide as a waste product.
McTighe, per a second embodiment, omits the central tube, flow connects the limestone tubes via ports near their top ends directly with the fuel bed in the furnace, and inserts an air blast pipe and a first vacuum pipe in the ash pit. A crude petroleum oil supply pipe is also inserted in the ash pit and a second vacuum pipe is inserted directly into the fuel bed for use in making illuminating gas. In this two phase cycle operation, first air is supplied from the air blast pipe to the bed to burn the fuel to heat to incandescence both the limestone batch, by indirect heat exchange, and the bed fuel, by direct combustion, with the combustion products exiting from the flue as in the first embodiment, and then the air supply is terminated and the flue is shut, after which a vacuum is applied via the first vacuum pipe to draw the liberated carbon dioxide from the limestone tubes via the tube ports through the bed of incandescent fuel for endothermic reaction therewith to form carbon monoxide which is recovered from the ash pit via the first vacuum pipe. When the bed is reduced by such reaction to a temperature below the working point, the first vacuum pipe is reclosed, the flue is reopened and the air resupplied from the air blast pipe to repeat the cycle.
To obtain illuminating gas in the McTighe second embodiment, during the carbon monoxide forming phase the first vacuum pipe is closed and the second vacuum pipe is opened while crude petroleum is supplied to the ash pit, whereupon the oil is vaporized and drawn through the bed along with the carbon dioxide from the tubes to produce a mixture of carbon monoxide and hydrocarbon gas which is recovered via the second vacuum pipe.
Since only indirect heat exchange is used to calcine the limestone batch in the tubes, very low thermal efficiency and very high fuel consumption per unit limestone throughput necessarily burden the McTighe system, and such negative aspects are aggravated by the extra fuel consumption demands for supplying heat for the separate suction induced carbon monoxide forming endothermic reaction, whether the latter consumes the coke or coal in the central tube per the first embodiment or the solid fuel of the bed itself in the furnace per the second embodiment. Essentially continuous consumption of fuel for heating the entire charge must be carried out until complete calcination is achieved, and at an accelerated fuel consumption rate under the suction induced flow in the system.
Niles (U.S. Pat. No. 1,798,802, issued 1931) calcines a batch of limestone, under vacuum in the presence of injected steam and by direct heat exchange, in a closed vertical shaft kiln at lower temperatures than otherwise, to avoid underdecomposing or overheating, recarbonation and semifusion, and instead produce a more uniform product. A bed of solid fuel in a fire box, surrounding and communicating with the calcining zone of the kiln, is supplied via an air fan with primary air, preheated by indirect heat exchange in flues surrounding the bottom discharge end of the kiln, and burned in the presence of steam injected both above and below the fuel bed, for direct heat exchange calcining of the charge, such that the spent combustion gases and liberated carbon dioxide are withdrawn from the top of the charge by a suction fan for recycling one portion to mix with the fresh combustion gases above the fuel bed in the fire box and for venting the remaining portion to the atmosphere to maintain the vacuum balance. The use of steam injection above the fuel bed to induce the recycled flow of the spent combustion gases and liberated carbon dioxide can replace the suction fan, and the use of steam injection below the fuel bed to induce the primary air flow can replace or be used in conjunction with the air fan.
In Niles, the steam injected below the fuel bed undergoes an endothermic water gas reaction with the solid fuel to form carbon monoxide and hydrogen (which thereby consumes heat and produces a cooling effect) and the latter in the presence of the primary air by way of a secondary reaction form carbon dioxide and reaction steam. This steam and the injected steam are said to decrease the partial pressure of the carbon dioxide and assist the release of carbon dioxide from the charge, whereas the recycled portion of the spent combustion gases and liberated carbon dioxide is said to cool and dilute the fresh combustion gases above the fuel bed for lower temperature operation in the kiln. The vacuum is stated to be minus four inches of water at the top of the kiln and two to three inches of water above the fuel bed in the fire box.
Obviously, in Niles the recycled carbon dioxide burdens the closed system as does that formed from the steam and solid fuel water gas reaction and the primary air supported secondary reaction, and the cooling effect of such recycled gases and that resulting from the water gas carbon monoxide reaction not only represent a reduction in thermal efficiency but, together with the need to supply heat for extraneous production of injected steam, also a concordantly higher fuel consumption per unit limestone throughput. Due to the intentional recycling of liberated carbon dioxide from the charge plus that in the spent combustion gases traceable to the combustion of the solid fuel in the fire box, which enrich the normal carbon dioxide content in the fresh combustion gases entering the calcining zone from the fire box, an undesirably high total content of carbon dioxide will build up in the closed system which will offset any benefits in reducing the carbon dioxide partial pressure by the injection of steam, except to the extent that a high vacuum load is utilized and a high proportion of the withdrawn gases from the top of the charge is correspondingly vented to the atmosphere, all of which will accelerate the fuel consumption rate and steam consumption rate, and unnecessarily add to the cost per unit limestone throughput. Continuous consumption of both fuel and steam for heating the entire charge must be carried out until complete calcination is achieved.
Hyde (U.S. Pat. No. 1,810,313, issued 1931) partially calcines a continuous supply of individual batches of limestone or other roastable material of mixed size lumps, preferably mixed with coal or other fuel, under vacuum and by direct heat exchange, on a continuously moving endless conveyor in a horizontal kiln. The batches are conveyed on pervious supports through a closed heating zone where fresh combustion gases from the burning of oil, gas or powdered coal in an adjacent fire box are drawn through the material via a suction box below the pervious supports and optionally recycled to a separate upstream preheater zone before discharge as spent gases. The partially calcined material batches are then conveyed to a separate closed, static soaking zone, not provided with heat or suction removal of gases, where calcining continues of the larger size lumps which have not yet completely reacted, until the material cools to below the reacting temperature, at which point the batches are conveyed out of the kiln.
In Hyde, the suction induced flow of the combustion gases through the heating zone necessarily accelerates the combustion rate of the fuel, and results in incomplete mixing and combustion of the air and fuel, nonuniform temperatures and especially inefficiently high fuel consumption per unit material throughput, whereas the static nature of the closed soaking zone undoubtedly causes the carbon dioxide pressure in the limestone material being calcined to build up and stifle the system, and in turn prematurely terminate the dissociation reaction and instead regress to extensive recarbonation, such that the material will be nonuniformly and incompletely calcined.
Walker I (U.S. Pat. No. 2,015,642, issued 1935) calcines a batch of limestone, under vacuum and by electrical heat, in a closed chamber, after exhausting the air therefrom, to obtain pure form carbon dioxide. The material is passed by gravity from a top preheater zone through an intermediate constricted passage furnace zone, having an upper electrical resistance preheater operating at 1500.degree.-1600.degree. F. (816.degree.-871.degree. C.) and a lower arc electrode heater operating at 2600.degree.-2650.degree. F. (1427.degree.-1454.degree. C.), to a bottom discharge zone, while a 10-15 lb. vacuum is applied to the top zone for recovering in pure form the carbon dioxide liberated in the furnace zone after it has preheated the material in the top zone and been itself cooled thereby. The use of such high temperatures for calcining completely the entirety of the limestone batch, and the continuous supplying of the necessary energy therefor from an electrical source, involve costs that are prohibitive under current world wide energy crisis conditions, regardless of economic conditions and electrical energy availability as of 1935, or the fact that the recovered pure form carbon dioxide constitutes a salable secondary product.
Walker II (U.S. Pat. No. 2,068,882, issued 1937) calcines a batch of limestone, in the presence of injected dry steam as carbon dioxide diluent and by electrical heat, in a closed vertical shaft kiln to obtain pure form carbon dioxide, the steam taking the place of vacuum. The kiln includes a top steam condensation and carbon dioxide gas recovery zone, in indirect heat exchange enclosing relation to a separate closed preheater hopper, and partitioned by a water collection tray having upright entrance nipples from an intermediate reaction zone containing an upper preheater section, a middle multiple distribution thin flow path calcining section and a lower soaking section. The latter zone is separated by a flow controlling valve from a bottom discharge zone from which the lime product is periodically recovered via a discharge valve. Dry steam at 900.degree. F. (482.degree. C.), i.e., well above the critical or condensation temperature of water (which is believed to be 705.2.degree. C. or 374.degree. C., at 217.5 atm or about 3200 psi), is injected under pressure separately into the soaking section and the discharge zone, part of the liberated carbon dioxide and steam as a mixed gas is drawn off by a recycling fan from the top of the calcining section, passed through a separate heating zone containing an electrical heater and then returned as hot mixed gas, e.g. 50% each, at 2000.degree. F. (1093.degree. C.) to the bottom of the calcining section for recycled flow upwardly therethrough, and condensed steam and pure carbon dioxide are recovered from the top zone by a water pipe and suction fan operated gas pipe, respectively. An auxiliary electrical heater is located in the discharge zone to keep the temperature therein above the 900.degree. F. (482.degree. C.) injected steam temperature.
In operating the Walker II kiln, a limestone batch, upon being preheated in the hopper by the condensing steam and separating carbon dioxide, which enter the top zone through the tray nipples at about 300.degree. F. (149.degree. C.) and are recovered therefrom in cooled condition, is charged to the reaction zone for passage as a thin stream in turn through preheater, calcining and soaking sections in countercurrent to the injected steam and recycled electrically heated mixed gas, and the soaked calcined product at 1600.degree. F. (871.degree. C.) is periodically fed to the discharge zone upon operating its valve. The steam injected into the soaking section cools the calcined product to about 1600.degree. F. (871.degree. C.) and is superheated thereby before mixing with the hot mixed gas at about 2000.degree. F. (1093.degree. C.) returned from the electrical heater, yet it is said that the cool lime even if it decreases below 1600.degree. F. (871.degree. C.) will not recarbonate or form hydrated lime due to the high temperature and thus dry condition of the steam, and that the steam injected into the discharge zone and the auxiliary heater therein similarly prevent such recarbonation or hydration.
In the Walker II system, the pressure of the injected steam is said to reduce the carbon dioxide partial pressure and force it upwardly through the kiln, and if the steam is in high proportion, e.g. 80% steam and 20% carbon dioxide, it can be used for recycling the mixed gas and recovering the carbon dioxide without the recycling fan or suction fan, the steam also serving to take up any hydrogen sulfide stemming from impurities in the limestone and dust formed during the calcining. However, the entire charge must be calcined to completion by continuously supplying electrical energy and pressurized steam throughout the residence time. Thus, like Walker I, the operation involves prohibitive electrical energy costs plus additional costs for also providing very high temperature steam, wholly apart from the costs of operating any fans or using the extra steam to achieve proper flow of the gases within the system.
Walker III (U.S. Pat. No. 2,113,522, issued 1938) concerns a related kiln system to Walker II, but instead of supplying pressure injected high temperature steam, the carbon dioxide recovery suction fan is used to induce a vacuum in the batch operated closed kiln, after removing the air therefrom, such that the liberated carbon dioxide alone serves as the inert heating medium recycled through the separate heating zone containing the electrical heater. This operation suffers from the same prohibitive cost drawback as Walker I and Walker II in requiring a continuous supply of electrical energy for calcining the entire charge of limestone until all of it has been converted to lime.
Haas (U.S. Pat. No. 2,080,981, issued 1937) calcines a periodic supply of limestone, under vacuum and by indirect heat exchange, in a closed continuous vertical tube whose intermediate and partially inclined portion is enclosed in a separate furnace, to obtain pure form carbon dioxide. The tube is equipped with synchronously operating gas tight rotary metering valves separating it in turn into an introductory chamber, a preheater chamber, an inclined calcining chamber, a reserve pocket and a cooling chamber. The preheater and calcining chambers are enclosed in indirect heat exchange relation within a gas or oil fired furnace having its own combustion gas venting stack, and the cooling chamber is supplied with cooling air via a rotary check valve containing inlet pipe. Also, each chamber (except the pocket) has a rotary check valve containing suction pipe with a suction fan downstream of the check valve, the preheater and calcining chamber suction pipes leading to a pure carbon dioxide collection system, and the introductory and cooling chamber suction pipes leading to the furnace to supply it with preheated combustion air, including the cooling air normally fed via the inlet pipe to the cooling chamber plus that entering the system from the atmosphere when a batch of limestone is periodically charged through the first valve into the introductory chamber and when a batch of calcined lime is periodically discharged through the last valve from the cooling chamber, in the latter case during those intervals in which the cooling air inlet pipe is closed off by its valve and prior to receiving the next increment from the reserve pocket so as to prevent air seepage upwardly through the pockets or sectors of the rotary metering valve between the cooling chamber and reserve pocket.
In Haas, as limestone is periodically charged to the introductory chamber any entering air is removed via its suction pipe, whereupon the charge is metered into the preheater and calcining chambers in turn, both within the furnace, such that the liberated carbon dioxide is recovered via their respective suction pipes while the furnace combustion gases are merely vented to the stack. The calcined lime is then metered into the pocket for final metering into the cooling chamber from which it is periodically discharged via the last valve after being air cooled. This operation is costly because it uses less efficient indirect heat exchange for calcining the limestone with the sensible heat of the combustion gases in the separate furnace which are immediately vented to the stack, and because the entire charge must be continuously heated until complete calcination is effected, wholly apart from expenses in providing synchronously operating rotary metering and check valves, and suction fans, to recover pure form carbon dioxide.
Vogel (U.S. Pat. No. 2,784,956, issued 1957) covers a more recent vacuum calcining suggestion, as compared to the relatively antiquated above discussed proposals, which avoids the prohibitive cost of electrical heating as in Walker I, II and III, and the inefficiency of indirect heat exchange with combustion gas as in McTighe and Haas, in favor of direct contact heating with such combustion gases as in Niles and Hyde.
While similar to Niles, Vogel instead uses a producer gas fed central burner in the closed vertical shaft kiln, a suction fan to vent the spend combustion gases and liberated carbon dioxide from the top of the kiln, and a waste gas recycling system between the preheating and calcining zones to draw off some of the combustion gases and liberated carbon dioxide thereat at 900.degree. F. (482.degree. C.) under 90 psi steam injected induced flow for first removing the dust content in a dust trap and then recycling the same, relatively dust free, at 700.degree. F. (371.degree. C.) admixed with the injected steam to the central burner for extracting otherwise lost heat values, but thereby adversely enriching the system with recycled liberated carbon dioxide. Temperature control is effected via dampers at the suction fan and recycling system under the surveillance of thermocouple temperature indicators in conjunction with the steam and producer gas flows, such that the limestone is calcined in the calcining zone at about 1800.degree.-2000.degree. F. (982.degree.-1093.degree. C.) in the presence of steam and under partial vacuum. Here also, fuel and steam must be consumed for the entire time that the limestone is calcined and until all of it is substantially converted to lime, and under accelerated induced flow through the kiln due to the presence of the spent gas venting suction fan, in a manner analogous to Niles and Hyde.
In each of the foregoing proposals, the entire charge of the carbonate material is calcined to completion wile under the continuous application of electrical or combustion heating energy thereto throughout the operation. While stationary vertical shaft kilns, despite the above drawbacks, can be readily provided as sealed systems with vacuum suction, it will be appreciated that due to the continuous movement of horizontal rotary kilns it would be difficult, if not impossible, to provide them as vacuum suction operated sealed systems at the high order of magnitude calcining temperatures contemplated.
Lastly, Kinkade (U.S. Pat. No. 3,527,447, issued 1970) teaches that a still hot (e.g. 125.degree.-150.degree. F. or 52.degree.-66.degree. C.) batch of already low temperature kettle calcined (e.g. at 250.degree.-500.degree. F. or 121.degree.-260.degree. C.) gypsum, i.e. calcium sulfate, which has meanwhile been quenched with liquid water to form the dihydrate, can be subjected to indirect heating at 190.degree.-200.degree. F. (88.degree.-93.degree. C.) under a reduced pressure of about 0.3 in.Hg absolute (29 in.Hg vacuum) in a vacuum chamber to remove sufficient water therefrom to form the hemihydrate (i.e. plaster of paris, a man made product which never occurs in nature) in a condition which is said to be more resistant to change on aging than previously manufactured man made hemihydrate. The heating of the vacuum chamber is necessary to keep the attendant water above its dew point so it will not condense on the walls of the chamber and recontact the hemihydrate and thereby reform the dihydrate, but at the same time it must be such that the formation of the undesired anhydrite by overcalcination is avoided.
Of course, the chemical system of low temperature partial or complete dehydration and rehydration of calcium sulfate, as regards water of crystallization, is completely different from the carbon dioxide partial pressure influencing temperature controlled dissociation and recarbonation system of calcium carbonate according to the equilibrium equation I. Indeed, calcium sulfate is often present as a lime impurity, traceable to precursor sulfur impurities in the starting carbonate rock and/or in the combustion fuel which are oxidized during the calcination operation, and no doubt exists as a dead burned inactive anhydrite slagging constituent as a result of calcining the rock at the usual limestone calcining temperatures.